A laser diode is used as a light source for various apparatus, including an image forming apparatus, an optical disc apparatus, and a communication apparatus, for example. The laser diode is controlled by a current output from a laser diode driving circuit 100 of FIG. 1, for example.
The background circuit 100 of FIG. 1, which drives a laser diode LD, includes a current supply 101, a first current mirror circuit 104, a second current mirror circuit 107, and a switch 108. As shown in FIG. 1, the first current mirror circuit 104 includes a pair of N-channel metal oxide semiconductor field effect transistors (“NMOS transistors”) 102 and 103. The second current mirror circuit 107 includes a pair of P-channel metal oxide semiconductor field effect transistors (“PMOS transistors”) 105 and 106.
An input current ia, provided by the current supply 101, is input to the first and second current mirror circuits 104 and 107, and output as an output current iLD to drive the laser diode LD.
Ideally, the output current iLD is proportional to the input current ia. However, in most cases, the input current ia and the output current iLD are not proportional to each other, depending on various factors including the channel length modulation effect, fluctuations in voltage, and variations in characteristic of the laser diode LD, etc. Accordingly, referring to FIG. 2, the characteristic of the background circuit 100, defined by the ratio between the output current iLD and the input current ia, is not always linear as indicated by an observed characteristic curve Lc, as opposed to an expected characteristic curve Lt.